Oxidation Ditch is usually made up of a circular or oval-shaped channel complete with mechanical aerators installed into the system. It is generally considered a modified system which is similar in working principles just like those in use for treatment of wastewater using activated sludge process. Similarly when flow enters the ditch; it will subject to heavy aeration and circulation (normally close to about 0.30 meters per second) thus allowing long solid retention times. The difference in this type of aeration basin is that there is a stream of returned sludge going back to the pond to mix with the inlet screened wastewater.
Design and Planning
The design criteria normally take the incoming feed into consideration whereby for it to function properly, the aerator unit must be able to provide uniformed dissolved oxygen delivered throughout the whole area and the ditch depth. For certain basin design, rotating biological contactors or RBC systems are used instead in placed of the aerators since it can help to increase DO level and preventing dead spot in the ditch that can lead to sludge accumulation and stagnant point. Therefore, this will allow a more thorough mixing and healthier growth of microbes.
Calculation of the pond surface area must be determined based on the detention time required and needed for the sludge process. For countries or regions that have the winter and summer conditions, both criteria have to be taken into account. Most oxidation ditches are built to treat incoming industrial wastewater with flow up to 3500 cubic meters per day and the BOD level can be in the range of 100 to 250ppm (maybe more depending on the capability of the aerator unit). Usually the final stage of the treatment system has sedimentation tanks built-in together with the unit for final water clarification purposes.
Operation and Maintenance
For optimal removal of BOD, ideally the MLSS level should be in the range of 3500 to 4500ppm but it can varies based on the overall aeration efficiency. Generally the wastewater going to the oxidation pond will mix together with the returned sludge and the rate of pumping and returned flow must be controlled in order to maintain that figure. Operation of the whole system also depends on the performance of sedimentation basin with good separation of sludge solid needed to achieve a clear effluent discharge. For the incoming wastewater going to the oxidation pond, it must be screened and have gone through grit chambers for particles removal in order to protect the mechanical equipment in the downstream process.